Research into areas such as collision avoidance, driving assistance, and automatic driving has been conducted in recent years, using technology for detecting objects around a vehicle with radar. Automobiles conventionally have radar on their front noses. A high-frequency oscillator needs to be provided in the vicinity of an antenna, and water and weather proofing measures, such as using a Radome for protection, are also necessary against wind and rain. Meanwhile, more sophisticated detection technology has also been developed, using both radar detection and camera images.
U.S. Pat. No. 8,604,968 proposes a radar-camera sensor in which a radar and a camera are housed in a single housing. The radar-camera sensor is mounted on the windshield of an automobile forward of the rear-view mirror. A radar wave is either a vertically or horizontally polarized radio wave.
International Publication No. WO/2006/035510 discloses an external-field-of-vehicle recognizing apparatus that is a multifunctional sensor unit in which an image capturing part and a transmission/reception part are mounted on a single sensor mounting board. The multifunctional sensor unit is installed in the interior of a vehicle.
When a radar device is provided in the interior of a vehicle, a radar wave is attenuated by being reflected and absorbed by the windshield. The influence of the glass increases if a short-wavelength radio wave is used to improve the resolution of the radar. Besides, it is not possible to increase the output of the high-frequency oscillator because there are statutory regulations governing the output of high-frequency oscillators available for use with vehicles. As a result, the distance that can be monitored by the radar is reduced.
European Patent No. 888646 discloses a method in which, when a communication antenna is installed in the interior of a vehicle, a dielectric intermediate element is disposed between glass and the radiating surface of an antenna in order to suppress the reflection of a radio wave on the glass. According to European Patent No. 888646, the electrically effective distance between the glass and the antenna is adjusted to several times the half-wavelength of the radio wave.
The windshield used in a vehicle such as an automobile is transparent and seemingly made of a single glass plate, but in actuality, they are made of three-layer laminated glass in which two sheets of glass are laminated on inner and outer sides of an extremely thin resin film, in order to ensure the safety of passengers. It has conventionally been thought that treating the windshield as a single glass plate, as in the case of viewing the windshield with visible light, would be enough to obtain accurate results. However, the inventor of the present invention has found this assumption to be wrong and recognized the need to focus on each layer of the laminated glass and take measures to suppress reflection. The present invention has been achieved on the basis of the idea of dividing the laminated glass into each layer and taking measures to suppress reflection on a layer-by-layer basis.